Study: lesson reminders on by default for new accounts; one-time backfill turns them on for existing accounts; v2.46.0

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/routes/study-auth.js

@@ -117,7 +117,8 @@ export function register(app) {
       passwordHash: hashStudyPassword(password),
       displayName,
       subscribeNewsletter: subscribe,
-      studyRemindersEnabled: false,
+      studyRemindersEnabled: true,
+      remindersDefaultedAt: now,
       pendingEmailChange: null,
       emailVerified: false,
       pendingEmailVerification: {